Practical Acoustic Treatment, Part 3

Tips & Techniques

PRACTICAL ACOUSTIC TREATMENT

PART 3: PAUL WHITE looks at how you can calculate how much studio
acoustic treatment you really need. This is the third article in a five-part series. Read Part 1, Part 2, Part 4 and Part 5.

Last month we discussed acoustic absorbers in some depth, but
since a successfully designed control room will use a combination
of absorption, geometry and scattering to produce the desired
acoustic environment, it would be wrong to regard absorbers as
the sole solution to the design problem. Absorbers are important,
however, in controlling the reverb decay time of the room. Other
surface treatments may be devised for the diffusion or scattering
of sound in order to further randomise the reflections arriving
at the listener, and this important area will be covered later
in the series.

FLUTTER ECHO

Flutter echo is a distinctive ringing sound caused by echoes bouncing
back and forth between hard, parallel surfaces following a percussive
sound such as a hand clap. To minimise flutter echoes, which can
plague even a studio having a perfect T60 across the band, certain
precautions should be taken. If you're building from scratch,
facing walls can be made out of parallel by at least 1 in 10,
but if this isn't possible, some form of mid/high absorber can
be applied to one or both walls to reduce the problem. In many
cases, a pair of acoustic foam tiles fixed to the side walls on
either side of the engineering position, as shown in the diagram
here, will be all that's needed.

Note that some of the absorbers discussed last month, such as
the panel trap, the Helmholz resonator and the slatted absorber,
have flat surfaces which are reflective at mid and high frequencies.
Consequently, when positioning these it's a good idea either to
face them with acoustic foam or not to have them opposite each
other across a parallel room. Alternatively, panel traps can be
constructed with a sloping surface, where the average depth is
maintained by making the halfway depth equal to the calculated
value. Padded door surfaces can also be beneficial; one of the
popular methods is to fit 2-inch foam to the door, then cover
this with upholstery-quality vinyl or fabric which is fixed by
tacks to give a quilted appearance.

Most of us will be familiar with reverberation, both as an artificial
and as a natural effect. It occurs in all normal rooms, to the
extent that music or speech sounds unnatural without it, but in
a studio control-room environment, reverb characteristics need
to be controlled within fairly close limits if the music produced
in the room is to be evaluated with any accuracy.

Reverberation is created whenever sound energy is fed into a room
and the room modes discussed the month before last are excited.
When the source of energy is removed, the reverberation will decay
at a rate determined by the geometry and absorbency of the room
and its contents. Excessive low-frequency reverberation related
to one dominating mode can cause serious problems for the engineer.
The danger is that you may attempt to correct your mix using EQ
to compensate for the apparent bass boost, but then when you play
back your mix on a properly balanced hi-fi system, the result
sounds bass-light. Furthermore, excessive reverb time at one frequency
can cause notes to hang on, generally blurring the sound and making
it more difficult to concentrate on fine details.

T60

Reverberation dies away exponentially, so some way of defining
decay time in a repeatable and measurable fashion is required.
Reverb time is conventionally defined as the time taken for a
sound to die away to one thousandth of its original sound level;
the resulting figure is called T60 (also known as RT60), because
the reverb time is measured to the point where the sound has decayed
by 60dB. The ideal reverb time varies depending on the room size
and the type of material being auditioned, though for a control
room it's likely to be around 0.3 seconds. In the studio area,
an optimum reverb time for speech might be somewhere between 0.2
and 0.5 seconds, whereas classical music might require between
0.6 and 0.8 seconds of reverberation to add life and body to the
performance. A typical living room has a T60 of around 0.5 seconds,
and -- unless you're going to spend a lot of money on studio design
-- that's not a bad figure to aim for in a project studio control
room. Some people would disagree with me on this point, but I
feel that unless you're doing the job properly, using qualified
designers who have access to the correct measuring equipment,
you can easily make the listening environment much worse by trying
to do too much.

In a poorly designed control room, problems arise because the
T60 tends to be different at different frequencies, though it's
normally OK to accept a longer T60 at lower frequencies, as is
typical of a furnished domestic room. The main thing to bear in
mind is that if the room isn't designed with low-frequency reproduction
in mind, it's invariably safer to use nearfield monitors with
a limited low-end response. Studio design ideals are subject to
changes in fashion, and no doubt the current interest in surround
monitoring will complicate the issue still further, but the current
consensus seems to be that, for small studio control rooms, we
should aim for as constant a reverb time as possible up to 8kHz
or beyond. Though a slight rise of reverb time at lower frequencies
is permissible, it should not be excessive.

SABINE

The maths needed to calculate reverb time or T60 is fairly straightforward
using the formula devised by WC Sabine at the turn of the century,

"If the room isn't designed with low-frequency reproduction in
mind, it's invariably safer to use nearfield monitors with a limited
low-end response."

though this formula is more accurate when applied to larger rooms
than to small ones. There is a more accurate and rather more complicated
formula attributed to Eyring, but in order to illustrate the basic
principles, Sabine will serve quite adequately. Sabine's formula
states that:

T60 = 0.05xV
STxAave

where T60 is the reverb time in seconds, 'V' is the volume of
the room in cubic ft, 'ST' is the total surface area of the room
in square feet and 'Aave' is the average absorption coefficient
of the surfaces within the room. Imperial measurements are used
here, but the metric equivalent is:

T60 = 0.161xV
STxAave

where the volume is measured in cubic metres, and the surface
area in square metres.

If the room is to be furnished, the surface areas, volumes and
materials of the furniture should really be included in the calculations,
but unless you're putting a lot of furniture into a small room
it's easier to do your calculations based on the empty room, and
then assume that adding any soft furnishings later will only improve
things. It's possible to obtain tables of absorption coefficients
relating to all the commonly used building, decorating and furnishing
materials (check a good builders' supply company and get leaflets
on specific materials for details), but a few useful ones gleaned
from various textbooks are included below. Keep in mind that these
can only be regarded as approximate, as no two manufacturers'
products are identical.

HERE COMES THE SCIENCE

Multiplying the total surface area of the room by the average
absorption coefficient for the surface materials tells us how
absorbent the room is, and this figure is expressed as so many
absorption units -- called 'Sabines'. Simply put, we can consider
each area of different surface material separately, calculate
the number of Sabines it contributes, and then add up all the
Sabines for the room to give us the bottom line for the simple
equation shown earlier. For example, assume that the absorption
coefficient for concrete at 125Hz is about 0.01, which isn't very
high. Five hundred square feet of concrete surface, such as a
floor, would give us 500 x 0.01 = 5 Sabines of absorption. Add
on the number of Sabines due to plaster walls, panel absorbers
or whatever, and you end up with the total number of Sabines for
the room at 125Hz.

To complicate the issue slightly, the absorption coefficient for
a given material varies with frequency, but it isn't practical
to do a different set of calculations for every possible audio
frequency. Instead, we rationalise the audio spectrum to six discrete
frequency values, at one octave intervals, from 125Hz up to 4kHz.
Even so, that means working through the formula six times with
six sets of values to give

"Ultimately, the only real imperative is that the room should
work for creating mixes that sound 'right' when played on other
systems outside the studio."

us six T60 times, one for each octave. Once this has been done,
the figures tell us at which frequencies we have either too much
or too little absorption. Then it's down to pencilling in a trap,
a carpet or a few acoustic tiles, and then going through the sums
again to see if things are better. Anyone capable of using a spreadsheet
program should be able to automate this tedious calculation, but
a simple calculator is quite good enough. Earlier in the series
I mentioned a software package called Acoustic X, by Pilchner-Schoustal (see screen shot, left), that does all
this for you. It also contains an extensive library of materials
and their coefficients, so it could make the job a lot easier.

One limitation of Sabine's equation is that it assumes a perfectly
diffused soundfield, which small rooms invariably don't have,
and it also ignores any sound absorption due to the air within
the room. That's another good reason why any result arrived at
on this basis should be treated as a guide rather than as a rigorous
analysis. Acoustic consultants make a good living out of weighing
the results of these and similar calculations against reality,
then applying their experience and expertise to come up with something
that will actually work.

DISTRIBUTED ABSORPTION

It's good practice to try to balance the properties of facing
walls, rather than calculating that you need a certain amount
of trapping for the whole room and then sticking it all in one
place. What's more, tuned trapping designed to combat specific
room modes must go on the wall relevant to those modes. For example,
if you have a mode which is due to the length of the room, the
trapping must go on the end walls, not the side walls. When treating
facing surfaces, it's most effective to distribute the absorptive
material between them, rather than putting everything on one wall
and leaving the other reflective; in the case of side walls, this
is essential to

"Studio design ideals are subject to changes in fashion, and no
doubt the current interest in surround monitoring will complicate
the issue still further."

maintain a nominally symmetrical listening environment. However,
it's not always possible to treat opposing surfaces in exactly
the same way, the floor/ceiling pair being the most obvious example.
If the floor is carpeted, it will absorb the higher frequencies
very efficiently but will hardly affect the bass or lower mid-range
at all. One answer might be to mount bass traps in the ceiling
to absorb the bass but to reflect back the mid and higher frequencies
absorbed by the carpet.

Decisions about where to place absorbers will also be influenced
by the underlying philosophy of the room. There are at least two
types of LEDE (Live End Dead End) control room, there are rooms
that rely heavily on scattering to diffuse reflections, and there
are very dead rooms driven by huge monitoring systems. Indeed,
there are so many design options that there will be a separate
article covering that topic later in the series, so don't start
sawing things up just yet. Ultimately, the only real imperative
is that the room should work for creating mixes that sound 'right'
when played on other systems outside the studio. Indeed, it is
sometimes argued that, as most music is listened to in a domestic
living room, we should model our control rooms on living rooms,
but the reality is that if we're to produce really good recordings,
we need a monitoring environment that's a little better than that
enjoyed by the listener. Whichever approach you take, the room
must be as acoustically symmetrical as possible about the monitor
system, and any large windows in the side walls should be balanced
by areas having similar acoustic properties on the opposite wall.

The design techniques are the same for the studio area as for
the control room, though you may decide on a longer T60 for the
studio, depending on the type of music you wish to record. Speech
requires a fairly dry environment, whereas acoustic instruments
thrive in a more lively setting.

DOING THE SUMS

Before getting down to working out the trapping for your room,
you should decide on the basic room philosophy. Most small studios
use a combination of diffusion and geometry to keep early reflections
from the speakers away from the listening position, combined with
trapping and diffusion on the rear wall, to prevent strong reflections
from bouncing directly back to the 'sweet spot'. However, this
is not the only approach, and the ideal solution will depend to
some extent on the shape and size of your room.

ACOUSTICS IN THE REAL WORLD

Few project studios are professionally designed and, to be perfectly
honest, even if you apply the basic formulae to calculate the
amount of absorbent trapping you need to add, the results are
unlikely to be precisely right. Part of the problem is that materials
never seem to behave exactly as their textbook values suggest,
and it's also well known that the way in which absorbent material
is distributed on the room's surface has a profound effect on
the outcome. Even so, a mathematical analysis of the requirements
should get you into the right ball-park, though I must emphasise
again that the final design should be verified by measurement,
after which further adjustments may be needed.

If the thought of wading through a load of calculations fills
you with foreboding, don't worry, because there are more empirical
approaches to acoustic design that can be applied by following
very general and well-proven principles, and these will be covered
later. Fortunately, you can tell a lot about the acoustics of
a room by listening to speech and music in that room, and even
if the design isn't quite as good as you might have hoped for,
the human hearing system is capable of compensating for a multitude
of sins providing it has some form of reference, such as well
mixed commercial music played over the same monitors.

Though you wouldn't go designing a professional studio using only
instinct and listening tests, you'd be surprised at how much you
can improve the performance of a typical home studio by adhering
to a few simple guidelines. One of the reasons why this works
is that the smaller monitors used in project studios don't have
the same extended bass as the main monitors used in typical commercial
installations, so there is less low-frequency energy produced
to excite the room where its T60 is longer than might be desirable.
In addition, smaller monitors can be used closer to the engineer,
so the ratio of direct to reverberant sound is higher, meaning
that the room acoustics have less of an effect on the perceived
sound.

Once you've decided on a layout for your room:

* Check the room dimensions to see if they fall inside Bolt's
area (see the graph above, first shown in the July '98 issue,
for more details).

* If they don't, plot out your main room modes and find out where
trouble spots are likely to occur, so that you can employ some
extra trapping if necessary. Even if the dimensions fall inside
Bolt's graph, it's a good idea to calculate the room modes anyway,
as you can still end up with trouble spots, especially in small
rooms where the low-frequency modes are more widely spaced.

* Next, decide very carefully on what floor covering is to be
used, as this will have a significant effect on the overall acoustics,
due to the large area involved. At this point, you could use Sabine's
formula to work out the T60s for the room as it currently is,
at the standard frequencies of 125Hz, 250Hz, 500Hz, 1kHz, 2kHz
and 4kHz. This will probably reveal an excessively long T60 at
125Hz, though if the walls are hard and reflective you'll probably
find the room is very live in the mid-range too.

* With the help of the Sabine formula and a table of absorption
coefficients for your room materials, you should be able to arrive
at the areas of treatment that will be required to get the T60
close to your target figure at all six frequencies, though don't
forget that tuned traps need to go on the walls relevant to the
modes they're trying to compensate for. The best way to do this
is to calculate how many Sabines you need to provide at each frequency
and then distribute them according to the room philosophy and
the most dominant room modes. Any surfaces not occupied by doors,
shelves, windows, equipment and so on may be used to distribute
your acoustic absorbers. Don't panic, though, because in a typical
domestic room the amount of acoustic treatment needed isn't usually
that great -- it's not as if you have to cover all the available
wall space with traps.

The procedure of calculating the amount of absorption required
at each of the six standard frequencies sounds more complicated
than it is, but it can be time consuming. What's more, the result
is only going to be an approximation, due to the limitations of
Sabine's equation when applied to small rooms, not to mention
the uncertain absorption coefficients of various materials. Furthermore,
the overall effect of the same area of absorbent material will
be different depending on whether the material is concentrated
in one place or distributed around the room. As a rule, distributed
absorption works more effectively, but careful listening or specialised
measurement is the only real way to determine whether you have
a successful result.